1. Field of the Invention
This invention relates to a process of making a yarn wherein a stream of drawn core fibers is twisted in a generally triangular twisting space defined by two juxtaposed, closely spaced apart twisting drums rotating in the same sense, and covering fibers are wound on said stream of fibers in said space, and to apparatus for carrying out the process.
2. Description of the Prior Art
It is desired to prevent an untwisting in a false-twisting sense of the fibers of a drawn roving which has been twisted in the triangular twisting space defined by two twisting drums which rotate in the same sense and to ensure an adequate cohesion of the twisted fiber structure. This can be accomplished in that covering fibers are wound around the drawn roving as it is twisted in the twisting space. The covering fibers to be wound around the roving are parallelized in that they are drawn by means of a drawing frame and are moved with a constraint on a drum, which is coaxial to one of the two twisting frums rotating in the same sense and to which a vacuum is applied on part of its circumference. From Austrian Patent Specification No. 377,018 it is known that said parallelization and constraint can be utilized to make a uniform yarn of high strength if the drum for feeding the covering fibers is driven at a surface speed which differs from that of the twisting drums so that the fact that the twisting drums are rotated at a surface speed which is sufficient for the twisting of the drawn roving but is generally higher than the speed which can be tolerated for the feeding of the covering fibers cannot adversely affect the uniform winding of the covering fibers around the drawn roving. Whereas the alignment of the covering fibers as they enter the twisting space is an essential requirement for the making of uniform yarns of high strength, the quality of the yarns depends also on a proper tying of the covering fibers between the fibers of the drawn roving. Such tying will be more effectively achieved if the covering fibers fly freely into the twisting space. Besides, a relatively expensive structure is involved in the provision of two drawing frames for the roving to be twisted and for the covering fibers, respectively. Moreover, the roving from which the covering fibers are obtained cannot readily be drawn the high degree which may be desired so that particularly thin yarns may comprise a relatively high proportion of covering fibers.